This invention relates to a lightning protection circuit particularly useful in processing plants for the production of aluminum and the like. The invention relates, more particularly, to a lightning protection circuit which involves the grounding of the neutral, center point of a direct current circuit in a processing plant and in preferred forms, to such a circuit in combination with safety circuits for the interruption of the grounding connection.
The production of aluminum by electrolysis of aluminum containing compounds is a very old and well known process. Commercial aluminum production may be carried out by the Hall-Heroult process in which aluminum oxide refined from bauxite ore is reduced electrolytically. Alumina, Al.sub.2 O.sub.3, the solute, is dissolved in molten cryolite, NaF/AlF.sub.3, the solvent, at a temperature of about 970.degree. C. The dissolved alumina, when subjected to a high intensity direct current in electrolytic cells, disassociates into positive aluminum and negative oxygen ions. In practice, a plurality of substantially identically constructed electrolytic reduction cells, for example, 112 cells are arranged in a pot line. That is, they are connected electrically in series. A direct current of from about 50,000 amperes to about 160,000 amperes or more, in commercial reduction systems, is not unusual. The electric path from the external direct current source, for example, a 600 volt source, is composed of the electrolytic bath within each cell, the cathode and anode structures thereof, conductive connections to these electrode structures and current buses which connect one cell to the next. The electrode structures and the baths exhibit a resistance through which the very high direct current of several thousand amperes flows. In practice, approximately a five volt D. C. voltage drop appears across each cell.
It has been determined that with no circuit connection from the direct current carrying circuit to earth ground, it was possible during electrical storm activity to build up static charges to a very high potential above earth ground in the power buses and cells. The build-up of the high potential static charges posed a distinct danger of arcing, with the possibility of danger to persons working in the pot room, to a computer used to control automatically the feeding of alumina to the reduction cells and to interface circuits which interconnect the direct current circuits to the computer.
Of the many possible problems encountered in a pot room in which an intentional earth ground connection can be used to provide some measure of lightning protection, two special problems have been encountered. The first of these problems is the occurrence of an accidental ground at a second point on the direct current power circuit. Unless the intentional ground is removed, large, undesirable ground currents will flow, depending on the location of the accidental ground. The second special problem encountered concerns the safety of personnel. If a person working in the pot room should become involved in an accidental connection to ground with the equipment he may be handling or through his person, the direct potential at that point of the direct current power system with which the person or his equipment is in contact would remain high; unless the intentional ground connection were removed. In this case, the floating neutral point would move to the accidentally grounded point thereby reducing the danger to the person working in the pot room.